Effect of exposing dentine to sodium hypochlorite and calcium

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Effect of exposing dentine to sodium hypochlorite and calcium
IEJ356.fm Page 113 Monday, January 29, 2001 2:53 PM
Effect of exposing dentine to sodium hypochlorite
and calcium hydroxide on its flexural strength and
elastic modulus
Blackwell Science, Ltd
D. Grigoratos1, J. Knowles2, Y-L. Ng1 & K. Gulabivala1
Departments of 1Conservative Dentistry and 2Biomaterials Science, Eastman Dental Institute for Oral Health Care Sciences,
University College London, London, UK
Abstract
Grigoratos D, Knowles J, Ng Y-L, Gulabivala K.
Effect of exposing dentine to sodium hypochlorite and calcium
hydroxide on its flexural strength and elastic modulus. Interna-
tional Endodontic Journal, 34, 113–119, 2001.
Aim The aim of this study was to evaluate the effect of
sodium hypochlorite (NaOCl) solutions (3%, 5%) and
saturated calcium hydroxide (Ca(OH)2) solution, individually and consecutively, on the flexural strength and
modulus of elasticity of standardized dentine bars.
Methodology Standardized plano-parallel dentine
bars (n = 121) were divided into five test groups and one
control group. The control group 1 consisted of dentine
bars, stored in normal saline until testing. The dentine
bars in the five test groups were treated by exposure to
the following solutions; group 2 – 3% NaOCl, 2 h; group
3 – 5% NaOCl, 2 h; group 4 – saturated Ca(OH)2 solution,
1 week; group 5 – 3% NaOCl, 2 h and then saturated
Ca(OH)2 solution 1 week; group 6 – 5% NaOCl, 2 h and
then saturated Ca(OH)2 solution 1 week. The dentine bars
were then loaded to failure in a three-point bend test.
Introduction
Sodium hypochlorite (NaOCl) and calcium hydroxide
(Ca(OH)2) have been employed in root canal treatment
principally because of their antimicrobial properties
(Shih et al. 1970, Stevens & Grossman 1983, Byström &
Sundqvist 1985, Safavi et al. 1985, Sjögren et al. 1991,
Stuart et al. 1991). Their additional properties include
the ability to denature toxins (Buttler & Crawford 1982,
Correspondence: K. Gulabivala, Department of Conservative Dentistry,
Eastman Dental Institute for Oral Health Care Sciences, University
College London, 256 Grays Inn Road, London WC1 8LD, England, UK
(fax: +020 7915 1028; e-mail: [email protected]).
© 2001 Blackwell Science Ltd
Results The data revealed a significant (P < 0.001)
decrease in the modulus of elasticity and flexural
strength of the dentine bars treated with 3% and 5%
NaOCl. There was no significant difference in the
flexural strength and the modulus of elasticity between
the 3% and 5% NaOCl groups. Exposure to Ca(OH)2
significantly (P < 0.001) reduced the flexural strength
but had no significant effect on the modulus of elasticity. The groups treated with sodium hypochlorite
followed by calcium hydroxide did not have moduli
of elasticity and flexural strengths that were significantly different from those treated only with sodium
hypochlorite.
Conclusions NaOCl (3 & 5%) reduced the modulus
of elasticity and flexural strength of dentine. Saturated
Ca(OH)2 reduced the flexural strength of dentine but
not the modulus of elasticity. Sequential use of NaOCl
and Ca(OH)2 has no additional weakening effect.
Keywords: calcium hydroxide, dentine, elastic modulus, flexural strength, sodium hypochlorite.
Received 22 December 1999; accepted 1 March 2000
Safavi & Nichols 1994) but more importantly, they can
both dissolve organic tissue (Grossman & Meiman
1941, Senia et al. 1971, Hand et al. 1978, Thé 1979,
Koskinen et al. 1980, Gordon et al. 1981, Abou-Rass &
Oglesby 1982). Furthermore, the combined action of
NaOCl and Ca(OH)2 has a synergistic effect on tissue
disintegration (Hasselgren et al. 1988, Morgan et al.
1991, Andersen et al. 1992, Shue-Fen et al. 1995,
Türkün & Cenzig 1997, Tatsuta et al. 1999).
Dentine is composed of approximately 22% organic
material by weight (Trowbridge & Kim 1991). Most of
this consists of collagen type I, which contributes
considerably to the mechanical properties of dentine. It
International Endodontic Journal, 34, 113–119, 2001
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Effect of NaOCl and Ca(OH)2 on dentine Grigoratos et al.
would be reasonable to assume that the dissolution
effect of NaOCl and Ca(OH)2 would affect dentine. It is
therefore pertinent to ask whether NaOCl and Ca(OH)2
weaken dentine.
Cvek (1992) queried the possible association between
long-term Ca(OH)2 dressing of traumatized anterior
teeth and their susceptibility to cervical fracture. He
reported finding no published evidence for mechanical
weakening of dentine as a result of such dressing. Lam
& Gulabivala (1996) found that exposure of teeth to
only 1% NaOCl rendered the dentine more susceptible
to removal by filing. More recently, it has been shown
that exposure of dentine to 5% NaOCl may reduce its
flexural strength and modulus of elasticity (Sim 1996)
when 0.5% had no significant effect. Sim (1996) further
examined the influence of irrigating root canals with
NaOCl (0.5%, 5%) on tooth surface strain when the teeth
were loaded and found a significant difference between
the two concentrations. The extent of this effect at other
concentrations of NaOCl (3%, 5%, 7%) was evaluated
by Goldsmith (1998) but the results were inconclusive.
The possible mechanisms involved in dentine depletion and consequently the weakening effect of NaOCl
have been investigated (Sakae et al. 1988, Barbosa et al.
1994, O’Driscoll 1999). Although Sakae et al. (1988)
and Barbosa et al. (1994) considered both the organic
and inorganic components of dentine to be affected,
O’Driscoll (1999) conclusively showed that only the
organic element was depleted.
The influence of Ca(OH)2 or the sequential use of
NaOCl and Ca(OH)2 on the mechanical properties of
dentine have not been reported. Given the widely recommended regimen of sodium hypochlorite irrigation
followed by calcium hydroxide dressing (Metzler &
Montgomery 1989, Sjögren et al. 1991, Ørstavik et al.
1991), it would seem appropriate to determine their
effect on the mechanical properties of dentine.
The aim of this study was to evaluate the effect of 3%
and 5% NaOCl and saturated Ca(OH)2 solutions on the
flexural strength and modulus of elasticity of standardized dentine bars. In addition, the effect of sequential
exposure to NaOCl (3% or 5%) and saturated Ca(OH)2
solutions on the flexural strength and modulus of elasticity of standardized dentine bars was also investigated.
This method has been previously used by Jameson et al.
(1994) and Sim (1996).
Materials and methods
The flexural properties of rigid or semi-rigid materials in
the form of rectangular bars, that break at comparatively
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International Endodontic Journal, 34, 113–119, 2001
Table 1 Groups and numbers of specimens
No. of
specimens
Test
Medium
Group 1 (control)
Group 2
Group 3
Group 4
Group 5
Group 6
Total
Saline
3% NaOCl
5% NaOCl
Saturated (S) Ca(OH)2
3% NaOCl + S. Ca(OH)2
5% NaOCl + S. Ca(OH)2
19
21
20
20
21
20
121
small deflection, can be determined by the use of a
three-point loading system utilizing central loading on
a simply supported beam (American Society for Testing
& Materials 1989).
Fabrication of dentine bars
Standardized plano-parallel dentine bars (1 mm × 1 mm
× > 11.7 mm) were cut from freshly extracted, intact
human teeth (stored in 4% formal-saline), using a
diamond saw (Exakt, GmbH, Nordestedt, Germany). The
dentine bars were randomly assigned to one control
group and five test groups as shown in Table 1. The
control group was stored in normal saline until testing.
Preparation of test solutions
Sodium hypochlorite (3%) solution was prepared by
diluting household bleach (3.8% available chlorine,
J. Sainsbury PLC, London, England, UK) with distilled
water. Sodium hypochlorite (5%) solution was prepared
by diluting a 15% solution (Jeyes Group PLC, Norwich,
England, UK) with distilled water. The concentration of
the solutions was verified by iodometric titration (British Pharmacopoeia 1973) immediately prior to use. The
saturated Ca(OH)2 solution was prepared by mixing
Ca(OH)2 powder (BDH Merck, Poole, England, UK) with
distilled water until undissolved solute remained at
37 °C for 8 h. The concentration was standardized to
1 g 1 mL−1.
Treatment of dentine bars with test solutions
Application of sodium hypochlorite
The dentine bars in groups 2, 3, 5 and 6 were placed
into four separate vessels, each containing 50 mL
NaOCl of the various test concentrations (3% for groups
2 and 5, 5% for groups 3 and 6). The test solutions in
© 2001 Blackwell Science Ltd
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Grigoratos et al. Effect of NaOCl and Ca(OH)2 on dentine
the four containers were agitated at 120 strokes min–1
in a shaking bath (Grant SS40-D Grant Instruments
Ltd, Cambridge, England, UK) for 2 h. The NaOCl solution was changed every 15 min to prevent saturation by
reaction products and to ensure that all surfaces of the
dentine bars would be exposed. The temperature of the
shaking bath was set at 37 °C. After 2 h, the dentine
bars were removed from the containers and any NaOCl
was neutralized by washing with five 50 mL changes of
distilled water in new containers. Groups 2 and 3 were
stored for testing in distilled water, whilst groups 5 and
6 were then treated with Ca(OH)2.
Application of calcium hydroxide
The dentine bars in groups 4, 5, and 6 were immersed
in the Ca(OH)2 solution, using three separate containers.
These were placed in a larger plastic vessel and the
relative humidity was maintained at 100% by placing
moist gauze around the small containers. They were
then placed in the shaking bath for a week. The temperature was set at 37 °C. The dentine bars were removed
from the containers and thoroughly washed with saline
until no visible traces of Ca(OH)2 remained on the
dentine bars. The dentine bars were stored in distilled
water until testing.
Three-point bend testing of dentine bars
The 121 dentine bars were subjected to three-point
bend tests, using a test-jig mounted on a load testing
machine (Hounsfield Ltd, London, England, UK). Each
bar was placed across the lower supports of the test jig
and loaded at the mid-point through the loading head
and shaft. All the dentine bars were kept moist during
testing with distilled water. The load testing machine
was run at a cross-head speed of 0.5 mm min–1 to failure.
Data were recorded on a plotter to give (PL3, JJ Instruments Ltd, London, England, UK) load-displacement
curves on graph paper. The load at fracture was recorded
directly from the load testing machine and verified against
the load/displacement curve.
Calculation of modulus of elasticity and flexural strength
E = (L3 m)/(4 b d3)
(key below).
Flexural strength
The flexural bend strength (FBS) was calculated using
the following equation:
FBS = (3 P L)/(2 b d2)
where:
E = modulus of elasticity in bending (Nm–2)
S = stress at fracture (flexural strength Nm–2)
P = load at the moment of fracture (N)
L = the support span (m)
b = the width of beam tested (m)
d = the depth of beam tested (m)
m = the slope of the initial straight-line portion of the
load deflection curve (Nm–1 of deflection)
(American Society for Testing & Materials 1989).
Statistical analysis
The raw data were tabulated and the means and standard
deviations calculated for each group. The data were checked
for normality, using Kolmogorov–Smirnov and Shapiro–
Wilk tests. Nonparametric tests (Mann–Whitney U-test)
were used to detect statistical significance (SPSS for
Windows 6, 1996 SPSS Inc, Chicago, IL, USA).
Results
Modulus of elasticity
The means and standard deviations of the modulus of
elasticity calculated from the raw data are presented in
Table 2. The Kolmogorov–Smirnov and Shapiro–Wilk tests
Table 2 Means and standard deviations (SD) of the modulus of
elasticity of the six groupsa
Modulus of elasticity (Nm–2)
Group
Test medium
Mean
SD
Group 1
Group 2
Group 3
Group 4
Saline
3% NaOCl
5% NaOCl
Saturated (S)
Ca(OH)2
3% NaOCl +
S. Ca(OH)2
5% NaOCl +
Ca(OH)2
5.2E + 10
3.0E + 10
3.5E + 10
4.8E + 10
1.1E + 10
1.1E + 10
9.7E + 09
8.6E + 09
3.8E + 10
1.5E + 10
4.3E + 10
2.6E + 10
Modulus of elasticity
Group 5
The slope of the tangent to the initial straight line
portion of the load deflection curve was drawn and the
modulus of elasticity was calculated using the following equation:
© 2001 Blackwell Science Ltd
Group 6
a
(E + 10 = 1 × 1010).
International Endodontic Journal, 34, 113–119, 2001
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Effect of NaOCl and Ca(OH)2 on dentine Grigoratos et al.
Table 3 Means and standard deviations (SD) of the flexural
strength of the six groupsa
Flexural strength (Nm–2)
Group
Test medium
Mean
SD
Group 1
Group 2
Group 3
Group 4
Saline
3% NaOCl
5% NaOCl
Saturated (S)
Ca(OH)2
3% NaOCl +
Ca(OH)2
5% NaOCl +
Ca(OH)2
8.7E + 07
3.7E + 07
4.9E + 07
6.1E + 07
2.5E + 07
1.9E + 07
2.3E + 07
1.6E + 07
3.2E + 07
1.6E + 07
3.4E + 07
1.5E + 07
Group 5
Group 6
a
(E + 10 = 1 × 1010).
showed that the data in groups 5 and 6 had a skewed
distribution. Nonparametric tests (Mann–Whitney Utest) were used to evaluate the significance of difference
in the modulus of elasticity between the different groups.
Analysis showed that there was a highly significant
(P < 0.001) reduction in the modulus of elasticity after
treatment with both 3% and 5% NaOCl. The difference
in the modulus of elasticity between 3% and 5% NaOCl
treated dentine was not statistically significant, although
the values for the 5% group were higher. There was
no statistically significant difference in the modulus of
elasticity between the controls and the Ca(OH)2 treated
dentine bars. The differences between Ca(OH)2 treated
dentine bars and those treated with 3% or 5% NaOCl
were statistically significant (P < 0.001). The dentine
bars treated with Ca(OH)2 had a higher modulus of
elasticity than those treated with either 3% or 5% NaOCl.
Statistical analysis showed that treatment of the dentine
bars with calcium hydroxide after their exposure to 3%
or 5% NaOCl, had no significant additional effect on
their modulus of elasticity.
Flexural strength
The means and standard deviations of the flexural
strength calculated from the raw data are presented in
Table 3. Groups 1–6 were checked for normality using
Kolmogorov–Smirnov and Shapiro–Wilk tests, and
groups 5 and 6 were found to have a skewed distribution. The Mann–Whitney U-test was used to evaluate
the significance of difference in flexural strength between
groups. Statistical analysis showed a highly significant
(P < 0.001) reduction in the flexural strength after
treatment with 3% and 5% NaOCl. The difference in the
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International Endodontic Journal, 34, 113–119, 2001
flexural strength of dentine bars treated with 3%
and 5% NaOCl was not statistically significant. Treatment of the dentine bars with the saturated Ca(OH)2
solution significantly (P < 0.001) reduced their flexural
strength. The flexural strength of the NaOCl (3%, 5%)
treated dentine bars was significantly lower (P < 0.001)
than the Ca(OH)2 treated ones. Statistical analysis
showed that treatment of the dentine bars with calcium
hydroxide after their exposure to 3% and 5% NaOCl,
had no significant additional effect on their flexural
strength.
Discussion
There was a striking difference in the mode of fracture
of the dentine bars, between the control group and some
of the specimens in the test groups. All the dentine bars
in the control group and most of the bars from the other
groups exhibited sudden ‘brittle’ fracture upon incremental loading with the fragments being ejected out of
the jig.
A small number of bars treated with Ca(OH)2, and
almost half of those treated with 3% and 5% NaOCl, and
those additionally treated with Ca(OH)2 did not exhibit
such fracture. Instead, the dentine bars appeared to
exhibit ‘green stick’ fracture without displacement from
the jig supports. In some cases, the specimens did not
fracture at the maximum load but continued deforming, fracturing at a lower load. This observed difference
in fracture patterns was corroborated by the load/
displacement plots. In the control group the plots
generally displayed a steep linear slope with little or no
disproportionate deformation prior to fracture. In the
Ca(OH)2 treated group, the pattern was similar to that
of the control group but the bars fractured at lower
loads and there was an obvious increase in the deformation of the specimens on loading. In a small number of
specimens, the upper part of the load/displacement
curve became nonlinear.
In the 3% and 5% NaOCl treated groups, there was a
considerable difference in the mode of fracture as
compared to the controls. The fracture loads were much
lower with considerable deformation of the dentine bars
prior to fracture. In groups where bars were treated
with Ca(OH)2 after exposure to 3% or 5% NaOCl there
appeared to be an even more drastic decrease in the
fracture loads, even though the difference did not reach
statistical significance. Particularly in those pretreated
with 5% NaOCl, all the loads recorded on fracture
were several orders of magnitude lower than those of
controls. Contrary to the trend observed in 3% and 5%
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Grigoratos et al. Effect of NaOCl and Ca(OH)2 on dentine
NaOCl, the deformation of the specimens remained
more proportionate prior to fracture. Just one specimen
in the 5% NaOCl and Ca(OH)2 treated group continued
displacing after the maximum load was reached. Slippage
during loading occurred in five specimens of each group.
Specimen no. 10 of group 5 did not fracture upon loading
but continued to deform. The testing had to be stopped
because no more data could be recorded and the crosshead approached its lower safety limit. The specimen
was removed in toto but separated on manipulation. The
fractured surfaces appeared to be ‘laminated’ and were
probably interlocked during the deformation.
There was a wide variation in the behaviour of the
dentine bars within groups, even in the untreated
group. This can be explained by the variety of teeth and
differences in their physical properties. Despite these
differences however, the influence of the treatment
rendered was dominant enough to show significant
differences (P < 0.001) between the groups. Treatment
of dentine bars with 3% and 5% NaOCl solutions both
caused a significant (P < 0.001) decrease in their modulus of elasticity and flexural strength but there were no
differences between these treated groups. The findings
are in accordance with those of Sim (1996). An unexpected finding was that the modulus of elasticity as well
as the flexural strength of the specimens treated with
5% NaOCl, were higher than those treated with 3%
NaOCl. Statistical analysis of the data however, did not
reveal this difference to be significant. A possible explanation of this effect could be the different origin of the
NaOCl solutions (Sainsbury’s household bleach and
Jeyes Group PLC).
Exposure of the standardized dentine bars to Ca(OH)2
significantly (P < 0.001) reduced their flexural strength
but had no significant effect on the modulus of elasticity.
Because Ca(OH)2 does not penetrate dentine well because
of the buffering capacity of hydroxyapatite (Wang &
Hume 1988), it is logical to assume that its effect is
more pronounced on the surface of the dentine bars
without considerably affecting the bulk of the dentine.
Treatment with Ca(OH)2 could thus potentially enhance
crack initiation and propagation on the surface of dentine
rendering it more prone to fracture. The central bulk
of the dentine would remain unaffected and so the
effect on the modulus of elasticity minimal. Further
treatment of dentine bars exposed to 3% and 5% NaOCl
with saturated Ca(OH)2 solution for 1 week, had no
statistically significant additional effect on their
modulus of elasticity and flexural strength. Nevertheless, the load/displacement plots do reveal differences
in behaviour which are not reflected in the measured
© 2001 Blackwell Science Ltd
outcomes. As this observation was unexpected, the design
of the study did not allow further analysis. A rational
explanation of the observations requires an understanding of the mode of action of both NaOCl and Ca(OH)2 on
dentine. The precise mechanisms of action are unknown,
leaving room for speculation.
It is evident from previous studies (Hasselgren et al.
1988, Andersen et al. 1992, Shue-Fen et al. 1995) that the
modes of action of these two agents are different. The
studies concurred that the tissue responded differently
to NaOCl and Ca(OH)2 in terms of physical appearance
as well as dissolution kinetics. They were also in agreement that Ca(OH)2 had a solvent effect; only Morgan
et al. (1991) found to the contrary. The concentrations
of the solute varied widely in these studies as did the
substrate under test, perhaps accounting for the differences. It should also be noted that the teeth in the
present study were fixed in 4% formal-saline and may
have reduced the dissolution effect expected in vivo (Thé
1979).
The synergistic effect so well demonstrated in other
studies (Hasselgren et al. 1988, Andersen et al. 1992,
Shue-Fen et al. 1995) was not obviously borne out
statistically by the outcome measures used, but observations did indicate such an effect. The encasement of
the organic component within a mineralized matrix
may account for the lack of direct correlation with
other studies. If both NaOCl and Ca(OH)2 act primarily
on the organic portion of dentine, then NaOCl may
deplete dentine to the extent that Ca(OH)2 may not
have any more accessible substrate left to exert its effect.
The depth of penetration of the solutions and pastes
should also be taken into consideration. Ca(OH)2 does
not penetrate dentine very well (Gomes et al. 1996),
so most of its effect will probably be limited to the surface, where the effect of NaOCl pretreatment will be
maximal. It should be noted that although the mean
flexural strength of the sodium hypochlorite-treated
dentine decreased after treatment with calcium hydroxide, the modulus of elasticity actually increased. It seems,
based on this observation, that further treatment
with calcium hydroxide – although weakening the
dentine – allows less deformation of the dentine prior
to fracture. This supports the concept of a different mode
of action for sodium hypochlorite compared with calcium
hydroxide.
It is evident that further studies should be undertaken
on the effect of NaOCl and Ca(OH)2 on the properties of
dentine. The reduction in the mechanical properties of
dentine may help to explain, in part, the observations
of cervical fractures in Cvek’s (1992) material.
International Endodontic Journal, 34, 113–119, 2001
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Effect of NaOCl and Ca(OH)2 on dentine Grigoratos et al.
Acknowledgement
Dr R. Holt’s guidance and advice on the statistical
analysis of the data is acknowledged.
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